Morphology and Microstructure Evolution of MHC Alloy Powder with HfC Doping during High-energy Ball Milling AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY24010020

Chinese Library Classification Number:TG146.4

Citation Information:

In the preparation of molybdenum alloys, carbides are considered to be excellent second-phase doping material due to their high hardness, high melting point, and excellent thermal stability, mainly due to the formation of fine, diffused high-melting point carbide phase by the addition of Ti, Zr, Hf or other active elements to react with C, forming a semi-co-lattice grain boundary with low energy in the molybdenum matrix, improving the binding force of the grain boundaries, to obtain the effect of diffusion and toughening. In this study, Mo-Hf-C(MHC) alloy bars were prepared by direct doping-powder metallurgy process, with second-phase doping material of HfC, which was a face-centered cubic structure with the highest melting point and the best thermodynamic stability among all carbides. Previous studies have shown that Vickers hardness of the prepared MHC alloy was 46.5% higher than that of pure molybdenum rods prepared by the same process. By the precipitation of the second phase HfC particles in the grain boundaries and grains, the dislocation activity of the alloy could be prevented, the grains could be refined, and the oxidation and volatilization behavior of matrix Mo element in MHC alloy could be inhibited to a certain extent due to the ultra-high melting point of doping HfC. At present, powder metallurgy was the widely adopted production process of molybdenum alloy. The physicochemical properties of the alloy powder used in the process had a significant impact on the quality of the subsequent molybdenum alloy, among which the morphology, particle size, and distribution characteristics of the second phase of the alloy powder were the basic indicators for characterizing and evaluating the alloy powder. The alloy powder prepared by a high-energy ball milling process had a uniform and fine microstructure, which had the advantages of a simple process and low energy consumption compared with liquid-liquid and solid-liquid doping and was a common method for preparing alloy materials. The factors such as the design of the high-energy ball milling process and the doping of the second phase would affect the milling effect. Due to the high hardness of doped second-phase HfC (Mohs hardness of 9) and the alloy mixture powder formed by the milling process being the basic raw material for the production of MHC alloy, the difference in the physical properties of the doped second-phase would inevitably affect the morphology, microstructure, and dispersion uniformity of the alloy mixed powder, and further affect the properties of the subsequent alloy. Therefore, it was of great significance to explore the influence of hard HfC doping on the characteristics of alloy powders by high-energy ball milling. To investigate the influence of hard doping on the morphology and distribution uniformity of alloy powder, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were adopted to analyze the morphology, phase, and element distribution of powder. The influence of hard doping on the preparation process of alloy powder was summarized. The results showed significant differences in the morphology and mechanical properties of pure Mo powder and HfC powder. Mo powder was a ductile spherical particle, whereas HfC powder was a brittle bulk particle. The particle shape of pure Mo powder was changed from the initial spherical shape to lamellar or lamellar peeling debris by different ball milling times, whose spherical morphology could not be inherited, while the shape of HfC powder could maintain the original bulk morphology after high energy ball milling. MHC alloy mixture powder belonged to the ductile-brittle powder system, and the second phase of hard HfC was uniformly dispersed in the alloy powder, so it could be used as an internal abrasive when mixed with Mo powder for ball milling. The spherical morphology of Mo powder was inherited due to the doping of hafnium carbide. Under the impact, extrusion, and stripping of the hard second-phase and agate ball, HfC powder obtained a better refinement effect than Mo powder. The ball milling process failed to achieve significant alloying. The alloy powder had changed from the initial uneven dispersion and stable state to an alloy mixture powder composed of equilibrium or non-equilibrium phases. Metallurgical welding occurred between the new surfaces so that the powder was in a high-energy state, which promoted the atomic-level mutual miscibility between the molybdenum powder and the second-phase particles, which was beneficial to the homogenization of the mass and the improvement of the strength of the sintered blank in the subsequent process. The optimal mixing and milling effect could be obtained by 5.5 h of forward and reverse rotation of the batch ball milling process with the addition of HfC as the second phase doping under the present ball milling condition.